1. Field of the Invention
The present invention relates to a sheet feeding apparatus incorporated into various types of equipment having a mechanism for feeding a sheet, such as a computer, copying machine, facsimile, word processor, typewriter and the like.
2. Related Background Art
In the past, as disclosed in the Japanese Patent Laid-Open No. 59-177243, such a sheet feeding apparatus was so designed that travelling waves were applied to elastic members pinching a sheet therebetween, thus feeding the sheet.
Now, a sheet feeding principle in this conventional sheet feeding apparatus will be explained with reference to FIG. 2.
A sheet 13 is pinched or interposed between elastic members 5 and 6 with an appropriate pressing force given by the elastic members. Advancing flexural vibrations (travelling waves) are applied to the elastic members 5, 6, respectively. Since it is designed that the phase difference between these flexural vibrations becomes 180.degree. spatially, the flexural vibrations travel or advance in the respective elastic members 5, 6 in such a manner that convex portions created in the respective elastic members and directing toward the interposed sheet 13 always face each other. In this case, in consideration of a certain particle on a surface of the elastic member 5 or 6 (for example, at the convex portion thereof), the particle generally moves along an elliptical orbit. In FIG. 2, regarding the elastic member 5, if the travelling wave advances to the right, the particle will move along the elliptical orbit in a clockwise direction, as shown. Accordingly, the particles on the convex portions of the elastic members 5, 6 all move in respective directions opposite to the advancing directions of the travelling waves in the elastic members 5, 6, respectively, thereby providing a force for feeding the sheet 13.
On the other hand, although a sheet feeding force created at concave portions of the elastic members acts in the same direction as the advancing direction of the travelling wave, since the pressing forces of the concave portions against the sheet are smaller than those of the convex portions, the friction forces between the elastic members 5, 6 and the sheet at the concave portions are also smaller, and, thus, the sheet feeding force created by the concave portions will be smaller than that created by the convex portions. Accordingly, the total sheet feeding force acts in the direction opposite to the advancing direction of the travelling wave.
FIGS. 3 and 4 depict sheet feeding apparatuses using such sheet feeding principle, where FIG. 3 shows an example that a pair of rollers are disposed at a sheet introducing side and FIG. 4 shows an example that the pair of rollers are disposed at a sheet ejecting side. In FIGS. 3 and 4, the reference numerals 1, 2, 3, 4 denote vibration elements each comprising a piezo-electric element acting as an electro-mechanical energy transducer (energy conversion element); 5, 6 denote the above-mentioned elastic members; 7 denotes an oscillator; 10 denotes an energy absorber; and CNT denotes a controller.
The vibration elements 1, 2 are fixedly mounted on the elastic member 5 by an adhesive and the like to constitute one set of vibrators. Similarly, the vibration elements 3, 4 are fixedly mounted on the elastic element 6 to constitute the other set of vibrators. Further, the elastic members 5 and 6 are urged against each other with an appropriate force, for example, by means of springs. In the illustrated embodiment, the elastic elements 5, 6 are made of conductive material and are connected to an earthing circuit or ground. By applying a frequency voltage (alternate electric field) to the vibration element 1 by means of the oscillator 7, the elastic members 5, 6 are vibrated. The vibration elements 2, 4 generate electric energy due to the vibration of the elastic elements 5, 6. The generated electric energy is dispersed by the energy absorber 10 including resistors and the like. Accordingly, the vibration of the elastic members is not reflected, but creates travelling waves in the elastic members. When the flexural vibration of each elastic member 5, 6 creates the travelling wave, a certain particle on the surface of the elastic member moves along an elliptical orbit. Accordingly, the outer portion of the flexural vibration always has a component of velocity directing toward a direction opposite to an advancing direction of the travelling wave. Since the sheet 13 always contacts with the outer portion of the flexural vibration, it is fed to the direction opposite to the advancing direction of the travelling wave. In this way, in FIGS. 3 and 4, the sheet is fed from right to left.
The reference numeral 8 denotes a rotary encoder; 11 denotes a roller fixed to a rotary shaft of the rotary encoder; and 9 denotes a roller. The roller 9 serves to urge the cut sheet 13 against the roller 11 to prevent the slip between the roller 11 and the sheet 13. As the cut sheet 13 is shifted, pulse signals are sent from the encoder 8 to the controller CNT. The controller CNT comprises a microcomputer and is designed to count the number of pulse signals from the encoder 8 for controlling the driving of the oscillator 7. Since it is so designed that the number of pulse signals of the encoder corresponding to a predetermined shifting amount of the sheet is previously set in the controller on the basis of an outer diameter of the roller 11 of the encoder 8 and the number of pulses per a revolution of the encoder, the controller can stop or deenergize the oscillator 7 when it receives a predetermined number of pulses. When the oscillator 7 is deenergized, the travelling waves are stopped, thus storing the sheet feeding force. Since the elastic members 5, 6 are urged against the sheet 13 with the appropriate force, when the travelling waves are stopped, a great friction force is generated between the elastic members and the sheet, thereby braking the sheet 13 which tends to still move by its own inertia. As mentioned above, since the braking force is great, the sheet can be stopped for a very short time substantially without overrun, and, therefore, the sheet can be positioned with high accuracy.
Further, a pinching force of the rollers 9, 11 for pinching the sheet therebetween was selected to be sufficiently great to prevent the slip between the roller 11 and the sheet 13 and to be substantially constant.
However, as mentioned above, since the relatively great and constant pinching force was applied between two rollers 9, 11 for detecting the feeding amount of the sheet 13 to prevent the occurrence of the slip between the roller and the sheet, there arose a problem that the sheet was difficult to be inserted between the rollers 9, 11 due to the reaction force from these rollers.